Knowledge of the epidemiology of Rett Syndrome is limited. Research has focused on the clinical, genetic and neuropharmacologic aspects of this perplexing disorder. The specific aims of Project 4 are: 1) to establish the Texas Rett Registry, a population-based registry of prevalent and incident case of Rett Syndrome who reside in Texas during 1988-1993, and 2) to conduct a population-based case-control study to search for parental, prenatal, perinatal and postnatal risk factors for Rett Syndrome. Texas Rett Registry. Texas' large and ethnically diverse population will enable the calculation of incidence and prevalence measures for a geographically defined area and for black, hispanic and white populations. After initial ascertainment of approximately 140 (prevalent or incident) cases of Rett Syndrome in the state of Texas during the first year of the registry, an estimated additional 10 incident cases of Rett Syndrome will be ascertained per year. Yearly follow-up of all registered cases of Rett Syndrome will ensure clinical and epidemiological longitudinal investigation of staging of the syndrome, health care, modes of treatment and quality of life. In addition, the registry will provide the incident cases for the case-control study as well as for the other 5 projects of this program project. Case Control Study. Approximately 40 incident cases identified by the Texas Rett Registry and 160 controls (case:control ratio of 1:4), identified via a density sampling strategy through the Texas Birth Certificate Registry, will be the subjects for this case- control study. Cases and their controls will be matched on birthdate. Parents of the subjects will be given a standard exposure history questionnaire to search for risk factors associated with Rett Syndrome. A matched analysis using multiple logistic regression to control for potential confounders will be performed. The resultant Mantel-Haenszel estimator (and 95% confidence intervals) of the population odds ratio will estimate the risk ratio of Rett Syndrome given various exposures. For biologically and statistically significant exposures identified in this manner, we will calculate their attributable risk percent for case control studies. This attributable risk percent will estimate the proportion of Rett Syndrome which could be prevented by eliminating the exposure. Risk factors, if identified, may provide important etiologic clues for Rett Syndrome.